京ற大学ේ災研究所年報 第47号C平成16年3月
Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No. 47 C, 2004

Study on the Effect of Coastal Forest to Tsunami Reduction

Kenji HARADA*, Yoshiaki KAWATA* COE Researcher, DPRI, Kyoto University

Synopsis
In order to utilize coastal forest as tsunami countermeasures, quantitativeevaluation for hydrodynamic effect of coastal forest was examined in this paper.
Relation between the forest density and the diameter of trunk is obtained through thefield measurement data of control forest and some previous researches, which is useful
information for the evaluation of effects. Numerical simulation including the model ofcontrol forest effect is performed for evaluating the quantitative effect for tsunami
reduction and damage. By using simulation results, effects of reducing tsunamis can bequantitatively evaluated in a relation to the tsunami and forest conditions.
Keywords: Coastal forest; Tsunami; Forest width; Forest density Tsunami reduction

1. Introduction
In recent years, demand to coast management is
expanded not only for disaster prevention but also forenvironment and utilization by a rise of theenvironmental consciousness and the citizen
participation in Japan. On the other hand, artificial
coastal barriers such as seawalls and breakwaters
have been constructed along the Japanese coast andhave played an important role in protecting thecoastal area from natural hazards such as tsunamis,
tidal waves and high waves. The artificial coastal
barriers have some problems, such as high cost of theconstruction and maintenance, modification of thepresent environment and inconvenience in utilizing
the coastal area. Therefore, the countermeasuresagainst tsunamis by only using the artificial coastal
barriers are not recommended for all coastal areasand in future coast of management. For more
appropriate management to reduce natural disasterand to keep good environment, it is required that a
new countermeasure method corresponding to eachcoastal area including the combination of artificial
and natural functions. One of new ways is to utilize acontrol forest along coast, which is traditionalcountermeasure for long time. However, quantitativeand concrete functions of coastal forest to reduce atsunami are not established and formulated, so that no
guidance to use control forest is available. In order to
use a coastal control forests more effectively as
countermeasures against tsunamis, it is important toevaluate the hydrodynamic effect of tsunami control
forest, and to further discuss a disaster preventionfunction. Therefore the present study aims to provide
the quantitative guidance of tsunami reduction effectfrom numerical simulation including the resistance oftsunami control forest.
2. Evaluation to the effect of tsunami reduction byusing of numerical simulation
2.1 Conditions of coastal control forest in Japan
In order to evaluate the tsunami reduction effectby the numerical simulation, some conditions ofcoastal forest, based on using actual forest conditions,should be selected. Harada and Imamura (2003)
summarized the relation between forest density andtrunk diameter from the field survey data of pine tree

forest in Japan. In general, the diameter of trunk isrelated to the amount of leaves, because the trunk
takes the role of pipe between the leaf and root.
Therefore, they control themselves and so the forest
density becomes small when the diameter of trunk
becomes large (Tanaka, 1998). By using this relation,
the diameter of trunk can be estimated from the forest
density. From Figure 1, the trunk diametercorresponding to the forest density are selected to 0.3
m and 10 trees /100m2, 0.15m and 30 trees /100m2,
and 0.1 m and 50 trees /100m2. Forest width is an
important parameter for tsunami reduction and it
varies from place to place. To examine the forest
width effect, forest width is selected to be 50, 100,
200, and 400m and the forest is put at a distances of
100m from shoreline. The tree height and the brunch †height are selected 10, 2m and the projected area rateof leaves is given to be 0.65 from field survey data
(Harada and Imamura, 2003). These conditions areused for the evaluation of tsunami reduction effect byusing tsunami numerical simulation. The conditionsof forest are shown in Table 1.
Shuto(1987)
Munakata(1991)
Aburaya(2000)
Original data

Trunk diameter(m)

0.5
0.4
0.3

volume of tree under water surface; V is the volume
of water; CM is the inertia coefficient. The relation of
the inundation depth and the vertical structure oftrunk and leaf change this drag coefficient. These
resistance coefficients include the effect of tree
structure. In the numerical simulation, these
resistance coefficients used to calculate the forest
effect quantitatively.

CD = 8.4

Vo+ 0.66VÊˆV
Á0.01 £ o £ 0.07˜
Ë
¯
V

(1)

CM = 1.7
2.3 Numerical simulation including coastal forests
In order to evaluate the effect of tsunami
reduction
quantitatively,
tsunaminumerical
simulation of run up including the resistance of thecontrol forest is carried out and the change ofhydraulic values on the land are examined. The
coastal landform for Sendai Bay is selected as an
example to discuss the effect of the control forest(Fig.2). To evaluate the tsunami reduction effects bycoastal forest, the tsunami height is selected to 1, 2
and 3 m and wave period is 10, 20, 30, 40, 50, 60 min.
When tsunami heights exceed 4 m, tree would start tobe broken (Shuto, 1987). In this simulation, trees
breaking can not be properly modeled. Therefore,
tsunami height is selected smaller than 4 m.

0.2
0.1
0.0
1

2

3

4 5 6 78102

3

4 5 6 782

100

Forest density(trees/100m )Fig. 1 Relation of forest density and diameter oftrunk (Harada et al, 2003)

Fig. 2. Conditions for tsunami numerical simulation

Table 1 Conditions of coastal forest for numericalsimulation
Forest density
10, 30, 50 trees/100m2
Trunk diameter
0.3, 0.15, 0.1 m
Forest width
50, 100, 200, 400 m
Tree height
10 m
Brunch height2m
Projected area rate
0.65

Table 2 Conditions of tsunami and landform fornumerical simulation
Tsunami height
1, 2, 3 m
Tsunami period
10, 20, 30, 40, 50, 60 min
Gradient of sea floor
1/200
Gradient of land
1/500
surface

2.2 Resistance of coastal forest in numericalsimulation
In the numerical simulation, the effect of coastal
forest is included as the resistance force inmomentum equation. Harada and Imamura (2000)
modeled the resistance force as drag and inertia
forces based on Morison’s equation and modeled theresistance coefficients of coastal forest against
tsunamis by using the results of hydraulic experiment.
Equations (1) are the resistance coefficients used bythem, where CD is the drag coefficient; Vo is the

The governing equation used for the numericalsimulation is the momentum equation (2) with thehydraulic resistance force by coastal forest (Harada
and Imamura, 2003).

∂M ∂ Ê M 2 ˆ∂h gn 2+ Á
˜ + gD + 7 3 M / M /∂t ∂x Ë D ¯∂x DC
AO M / M /V ∂M+ D+ CM O=0
2
2 Dx ⋅ Dy D
V ∂t

†

(2)

Elevation (m)4Without forest

20
Tsunami hight =2m
Period = 10 min

Elevation (m)

-2
4

-1000
0
1000
WithDistanceforest from shore line : x(m)20Width=200m
Forest Density= 30 trees/100m2

-2

-1000
0
1000
Distance from shore line : x(m)Fig.3 Calculated results without and with forest
2.5 Effect of Forest Width to Tsunami Reduction
The spatial distribution of the maximum
inundation depth, current and hydraulic forces areshown quantitatively from the calculated results.
Hydraulic force is expressed by the product of fluid
density, the inundation depth and the square ofcurrent, and used for the estimation of house damage(Hatori, 1984). Figures 4 show the spatial distributionof the rate of maximum inundation depth, current andhydraulic force in the case of forest width of 0, 50,
100, 200, and 400m, fixed the wave height 3m, thewave period 10 min and the forest density 30
trees/100m2, and the reduction rate of maximum
values are defined by equation (3).
r (Reduction rate of max. values)=†(max. value with forest)(max. value without forest)(3)

Front ofthe forest

1.2
1.0
0.8
0.6
0.4
0.2

50m
100m
200m
400m

1.6

Reduction rate of max. current: r

TsunamiHeight =3.0m

1.4

0.0

1.4
1.2
1.0
0.8

0
200
400
600 800
Distance from Shore Line : x(m)1.6
Tsunami
1.4 Height=3.0m
1.2 Front of
the forest
1.0
0.8
0.6

100m

TsunamiHeight=3.0m
Front ofthe forest

W=50m
W=100m
W=200m
W=400m

50m
100m

0.6
0.4
0.2
0.0

Reduction rate of max. hydraulic force: r

2.4 Results of numerical simulation
Figure 3 shows the calculated waveforms
without and with the coastal forest. From Figure 3,
tsunami is reflected at the coastal forest and reduced
the inundation depth and the run-up distance behind
coastal forest. In particular, when the inundation
depth is over the leaf and brunch height, thereflection effect on seaside and the reduction effecton inland side become large. The large resistanceforce of the leaf and brunch, which is calculated fromequations, causes these tsunami reduction effects.

1.6
Reduction rate of max.inundation depth: rwhere: M is the flux discharge; D ,the total depth, h,
the water level; n, the Manning roughness coefficient.
AO, the projected area of trees under the water; Dx, Dy,
the grid spatial size; and g, the gravitational
acceleration. The grid spatial size are selected as Dx= Dy =50 m, and time step is Dt =1 sec.

200m
400m

0
200
400
600
800
Distance from Shore Line : x(m)

200m

W=50m
W=100m
W=200m
W=400m
400m

50m

0.4
0.2
0.0
0
200
400
600
800
Distance from shore line : x(m)Fig. 4 Spatial distribution on reduction rate ofinundation depth, current and hydraulic force (Forest
density: 30 trees/100m2, Trunk diameter: 0.15m,
Tsunami height: 3.0m)
The front of the coastal forest is set to be at
100m distances from shoreline and the back is variedwith its width. Figures 5 show the relation with forest
width and reduction rate just behind forest. From
Figures 4 and 5, the reduction rates of inundation
depth just behind the forest are decreased from r=0.86 with forest width 50m to 0.18 with forest width
400m. These inundation depths are decreasing with afunction of the forest width (see Figure 5). Thismeans that the increasing of forest width can reducethe inundation damage at the back of coastal forest,
which related to inundation depth. The reduction
rates of current just behind the forest are decreased
from r =0.54 with forest width 50m to 0.24 with
forest width 400m. The current at the front isdecreased due to the reflection at front side of forest,
and the current at the back is reduced by thereflection and the energy loss passing throw forest.
As the hydraulic force is defined by the products ofthe square of current and the inundation depth,
hydraulic force is influenced by the current strongly
rather than the inundation depth. Its spatial
distribution inside the forest in Figure 4 isindependent of each forest width, dependent of theforest resistance force. The increasing of forest width
can reduce the house damage, which related tocurrent and hydraulic force. From these results ofnumerical simulation, it can be shown that the effects
of forest width increasing are large to reduce tsunami
inundation depth, current and hydraulic force. By
using these numerical simulation results, the effect of
coastal forest to tsunami reduction can be evaluatedTsunami period
10 min0 100 200 300 400 500
Forest width (m)
1.0
0.8
0.6

1.0
0.8

0.4

0.2

50m
Front of the forest
Tsunami Height=3.0m

Reduction rate of max. current:r

1.2

0.6
0.4

0 100 200 300 400 500

0 100 200 300 400 500

Distance from shore line : x(m)Distance from shore line : x(m)

0.0

1.6

0 100 200 300 400 500
Forest width (m)1.4Tsunami height
1m
2m
3m
TsunamiPeriod 10min

1.2

Forest density
2
10(trees/100m )230(trees/100m )250(trees/100m )Tsunami Height=3.0m

1.0

50m

0.8

0.4

0.6

0.2

0.4
0 100 200 300 400 500

0.0Distance from shore line : x(m)0 100 200 300 400 500
Forest width (m)Fig. 5 Effect of forest width to tsunami reduction(Tsunami period 10 min)2.6 Effect of forest density to tsunami reduction
In this section, the effect of forest density to
reduce tsunami is discussed. Figure 4 shows thespatial distribution of maximum inundation depth,
current and hydraulic force with forest density 10,
30,and 50 trees/100m2, fixed tsunami height 3m, andthe fixed forest width 50m. The values of them are
made non-dimension by the case without the forest(Equation 3). The reduction rates of inundation depth
just behind of forest (x=150m) are about r =0.8 ineach forest density case of 50m forest width.
Therefore it can be said that the effect of forest
density to reduce inundation depth is small. The
reduction rate of current is decreasing with theincrease of forest density and this rates are r=0.6,
0.54, 0.51 in the case of 10, 30, 50 trees/100m2 atjust behind of forest. However the effect of forest
density to reduce current is not large. As same asinundation depth and current, the hydraulic force atjust behind of forest is decreasing with the increase offorest density. Figures 7 show the relation with forest
density and reduction rate.
The fluctuation of
reduction rate due to forest density in same forest
width is smaller than the effect of forest width.
From these results of numerical simulation, it can beshown that the effects of forest density based on
actual forest condition are not large to reduce tsunami.

Fig. 6 Spatial distribution on reduction rate ofinundation depth, current and hydraulic force (Forest
width: 50m, Tsunami height: 3.0m)
1.0
0.8
0.6
0.4
0.2
0.0
0
20
40
60
2
Forest density (trees/100m )1.0
0.8

Reduction rate of max. Current: r

0.0

1.4

0.6

0.4

Forest density
2
10(trees/100m )1.4
230(trees/100m )2
1.250(trees/100m )Tsunami Height=3.0m
1.0
50m
0.8

Reduction rate of max. hydraulic force: r

0.2

0.6

1.6

Reduction rate of max. hydraulic force: r

0.4

0.8

Tsunami height
1m,
2m
3m
Tsunami Period
10min

Reduction rate of max.inundation depth:r

0.6

1.0

1.6

Reduction rate of max. inundation depth: r

0.8

Tsunami height
1m,
2m
3m

Reduction rate of max. current: r

1.0

Reduction rate of max. hydraulic force: r

Reduction rate of max. inundation depth: rquantitatively. The quantitative evaluation of tsunami
reduction would be done in the later section.

1.0
0.8

Forest width
50m,
100m
200m,
400m

0.6
0.4
0.2
0.0
0
20
40
60
2
Forest density (trees/100m )Forest width
50m,
100m
200m,
400m

0.6
0.4
0.2
0.0
0
20
40
60
2
Forest density (trees/100m )Fig. 7 Effect of forest density to tsunami reduction(Tsunami period 10 min)2.7 Effect of tsunami period to tsunami reduction
In order to examine the effect of tsunami period
to tsunami reduction, the relation with tsunami period
and reduction rate are shown in Figure 8. When thetsunami period become long, the tsunami reduction
effects become small. In the short period; 10 min, theTsunami height=3m

1.0

Forest width
50m
100m
200m
400m

0.8

0.8

Reduction rate of current: r

1.0

0.6

0.6
0.4

Forest width
50m
100m
200m
400m

0.2
0.0
0

20 40
60
Wave period (min)Reduction rate of hydraulic force: r

Reduction rate of Inundation depth: r

reduction effect becomes small caused by theinundation depth, which smaller than leaf and brunchheight. From these results, it can be shown that thetsunami reduction effect by coastal forest become
small to long period tsunami.

1.0

0.4
0.2
0.0
0

20 40
60
Wave period (min)Forest width
50m
100m
200m
400m

0.8
0.6
0.4
0.2
0.0
0

20 40
60
Wave period (min)Fig. 8 Effect of tsunami period to tsunami reduction(Tsunami height =3.0m)
2.8 Quantitative evaluation of forest to tsunami
reduction
The relationship of the coastal forest damage andthe tsunami intensity or the tsunami height issummarized in Table 3 from the old tsunami disaster
reports by Shuto (1992). This table shows that thecoastal forest can stop drifts in the case of theinundation depth smaller than 4 m, or tsunami
intensity is smaller than 2. However, Table 3 does notinclude the quantitative information for the design ofthe control forest and the effect of forest condition totsunami reduction. In this section, the quantitative
effects to tsunami reduction by coastal forest are
discussed by using the results of tsunami numericalsimulation. Table.4 shows the reduction rates ofmaximum run up distance, inundation depth, current
and hydraulic force just behind the forest. In order toevaluate the effect against tsunami damage, thesehydraulic values are evaluated for 10 min tsunami
period. From the Section 2.5, the effect to tsunami
reduction is influenced strongly by the forest width.
So the effects are evaluated for the forest width andthe tsunami height. As the mature forest conditions,forest density and diameter of trunk are selected 30
trees/100m2 and 0.15m.

Table 3 Relation between tsunami intensity andtsunami damage (Shuto, 1992)
Tsunamiintensity
Tsunamiheight (m)Coastalcontrol
forest

0

1

2

3

4

5

1

2

4

8

16

32

PartialdamageStopdriftsCompletedamage,No reduction
effectMitigate
damage,Stop drafts,
MitigatetsunamiFrom Table 4, the run up distance is decreased
from 0.86 to 0.57 by the increase of forest width andtsunami height. And the inundation depth is also
decreased from 0.86 to 0.50 by the increase of forest
width and tsunami height. These results are the same
results of Aburaya (2000). For further understandingof the effect of damage reduction by tsunami flow,
current and hydraulic force would be evaluated from
calculated result. The current is decreased from 0.71
to 0.34 and the hydraulic force is decreased from 0.53
to 0.01 by the increase of forest width and tsunami
height. However the reduction rates of currents andhydraulic force are larger than run up distance andinundation depth. Additionally, the reduction effectsof hydraulic force are larger than the inundation
depth and the current, which indicate the mosteffective mitigation to the tsunami. This means thatthe reduction effect of coastal forest is larger onhouse damage than on inundation damage. Table 4
could compensate quantitatively the effect of coastal
forest to tsunami reduction of Table 3 (Shuto, 1992).
Additionally Table 4 could evaluate the effect of
coastal forest to reduction tsunami damage by thecurrent. Moreover, Table 4 can be used for aquantitative standard of the tsunami reduction effects
by coastal forest for the coastal forest planning.
Table 5 shows the evaluation table of forest
effect, which is converted from Table 4. The
reduction rates are categorized by three stages, 0.990.7 as “a little effective”, 0.69-0.4 as “effective” and0.39- as “much effective”. The run-up distance andthe inundation depth are considered as the tsunami
reduction, and the current and the hydraulic force areconsidered as the damage mitigation. From Table 5,
it can be said that the forest width is required to be
200 m or more in order to reduce tsunami, the run-up
distance and the inundation depth to less than 70%.
And the forest width is required to be100 m or morein order to mitigate tsunami damage and current toless than 40%. By using these calculated results oftsunami reduction, the effects can be evaluatedquantitatively related to the tsunami and forest
conditions. However, the numerical simulation
results in this section are calculated in the limited
conditions of the tsunami period and the landform. In
order to plan the costal forest for tsunami
countermeasure, it is needed to further understand thecostal forest effect related to the tsunami period andthe landform.

Table 4 Tsunami reduction effect by coastal forest
1
2
3
Tsunami height（m）
Coastal control forest
Mitigate damage,(Shuto, 1987)
Stop drafts, ReducetsunamiRun upForest 50m
0.98
0.86 0.81
distance
width 100m 0.83
0.80 0.71
200m 0.79
0.71 0.64400m 0.78
0.65 0.57
Inundation Forest
50
0.86
0.86 0.82
depth
width
100
0.76
0.74 0.66
200
0.46
0.55 0.50
400--0.11 0.18
Current
Forest50
0.71
0.58 0.54
width
100
0.57
0.47 0.44200
0.56
0.39 0.34
400--0.31 0.24
Hydraulic Forest
50
0.53
0.48 0.39
force
width
100
0.33
0.32 0.17
200
0.01
0.13 0.08
400--0.02 0.01
Table 5 Forest effect to tsunami reduction
Tsunami height (m)1
2
3TsunamiForestA little effective (run-up,
reduction
width
inundation depth: 90-70%)< 200 m
200 m<
Effective(run-up,
Forestinundation depth: 70-40%)
width
TsunamiForestEffective (current: 70 damage
width
40%)
mitigation<100m
Much effective (hydraulicforce: 70 - 40%)100m <
Much effective (current,
Foresthydraulic force: 70 - 40%)width
3. Conclusions
The coastal forest conditions in the actual field
are complied to obtain the relation between the forest
density and diameter of trunk. Its diameter can be
estimated from the forest density by using of thiscorrelation, which is useful for the plan to coastal
forest condition. The numerical simulation with
forest model is carried out to evaluate the quantitative

effect of tsunami reduction, and the results are
summarized in some tables. This can give somequantitative information for the reduction effects ofcoastal forests. However, the results in this study arecalculated in the limited conditions of the tsunami
period and the landform. In order to plan the costal
forest for tsunami countermeasure, it is needed tofurther understand the costal forest effect related tothe tsunami period and the landform.

Acknowledgments
This research was supported by the Japanese
Ministry of Education, Culture, Sports, Science andTechnology (MEXT) 21st Century COE Program for
DPRI, Kyoto University (No.14219301, Program
Leader: Prof. Yoshiaki Kawata).
References
Aburaya, T (2000). Tsunami reduction effect of thecoastal control forest along Sendai Coast,
Graduation thesis of Tohoku University, 60p (InJapanese).
Harada, K. and Imamura, F. (2000). Experimental
study on the resistance by mangrove under theunsteady flow, Proceeding of the 1st Congress of
APACE, 975-984.
Harada, K. and Imamura, F. (2003). Evaluation oftsunami reduction by control forest and possibilityof its use for mitigation, Proceedings of coastal
engineering, JSCE, 341-345 (In Japanease).
Hatori, T. (1984). On the damage to houses due to
Tsunamis, Bull. Earthquake Research Inst. Univ.
Tokyo, 59, 433-439 (In Japanese).
Munakata, H., Ohtsuki, K., Arai, S. and Yanagida, N.
(1991). Study on the disaster prevention coastal
forest, Research report of Fukushima prefectual
foresty experimental staion, 29, 1-12 (In Japanese).
Shuto, N. (1987). The effectiveness and limit oftsunami control forests, Coastal Engineering inJapan, 30, 143-153.
Shuto, N. (1992). Tsunami intensity and damage,Tsunami engineering research report, 9, 101-136 (InJapanese).
Tanaka, K. (1998). Survey the forests density, 50
methods for survey forest, Japan forest Technical
Association, 74-77 (In Japanese).

要

旨

海岸林を津波ේ災対策に活用させるため，本研究では海岸林の流体力学的影‫؜‬による津波低減効果を定量的に評価・検討した．評価する際に重要となる樹林密度と胸‫ݗ‬直径の関係は，入手できた現地観測結果から条件০定を行った．これを用い，海岸林の影‫؜‬を考慮した数値‫ڐ‬算を行い，津波の低減効果を定量的に評価・検討を行った．
その結果，いくつかの津波と海岸林の条件による津波低減効果を定量的に検討することのできる表を示した．
キーワード: 海岸林，津波，林帯幅，樹林密度，津波低減効果

COE-11
津波災害低減のためのේ潮林（自然力）の活用について

河田惠昭・〇原田賢治
１．はじめに

４．ේ潮林の活用について

低頻度で広域にわたる津波の対策として，全沿岸

津波はේ潮林の間を通過することができるので，にସ大な構造物による対策をとることは不可能であේ潮林だけでは陸上遡上を全てේぐことはできない．

る．しかし，物的津波被害の軽減のためには津波のしかしේ潮林をひとつの対策ツールとしてとらえ，進入を最小限にとどめる必要があり，構造物に加え構造物による対策等と組み合わせることで，ේ潮堤

ේ潮林などの自然力も活用した外力低下の対策が必等の構造物の想定津波以上の津波に対しても多段的要である．従来，津波対策において付加的要素としな対策をとることができ，津波遡上を低下させ被害てしか着目されてこなかったේ潮林のේ災機能の再を最小限にすることが可能である．ේ潮林の活用に評価により，ේ災・環境・利用に配慮した海岸整備はこのような組み合わせによる対策を地域の津波ේ

のひとつのツールとしてේ潮林を活用することがで災システムの中に取り込んでいく必要がある．具体

きる．ここではේ潮林を考慮した津波数値シミュレ

例としては，大ற市周辺の臨海埋め立て地に見られーションを用いて，津波・ේ潮林の条件による津波

る未利用地を活用し，埋め立て地周辺の堤ේとේ潮低減の特徴について検討を行う．また，ේ潮林の活林をற市公園の一環として整備することにより，ේ用についても述べる．

災・環境・利用に配慮したේ潮林の利用を期待する

２．津波数値‫ڐ‬算ことができる．

津波周期を変化させて数値‫ڐ‬算を行った．ේ潮林の抵抗係数は水理模型実験に基づく抵抗係数モデル（原田,2003）を用いている．
３．ේ潮林による津波低減の特徴‫ڐ‬算結果の整理の例として，Fig.1,2 にේ潮林による津波低減率とේ潮林幅，津波周期の関係を示す．

Tsunami height
1m,
2m
3m

0.8
0.6
0.4
0.2
0.0

Tsunami period
10 min0 100 200 300 400 500
Forest width (m)

津波の浸水深の低減率はේ潮林幅と比例関係にある

また，津波周期がସくなると浸水深の低減率は大きくなる傾向にあるが，周期 20 分以下でේ潮林幅200m 以下の時には周期が短い方が低減率は大きくなる傾向が見られる．これらの検討結果よりේ潮林
による津波低減効果を定量的に推定することが可能となり，ේ潮林の有効性が示された．

1.0

Reduction rate of Inundation depth: r

関係する流速は 4~6 割に低下させることができる．

Tsunami height
1m,
2m
3m
Tsunami Period
10min

0.8
0.6
0.4
0.2
0.0

0 100 200 300 400 500
Forest width (m)Fig.１ේ潮林幅と津波の減衰効果の関係事がわかる．200m のේ潮林幅ならば，浸水被害に関係する浸水深を 5~6 割に，流体力による被害に

1.0
Reduction rate of max. current: r

方程式に取り込み，ේ潮林幅，樹林密度，津波‫ݗ‬さ，

1.0

Tsunami height=3m

1.0

Forest width
50m
100m
200m
400m

0.8

0.8

Reduction rate of current: r

により差分化して行った．ේ潮林を抵抗として運動

Reduction rate of max. inundation depth: r

津波数値‫ڐ‬算は೪線形浅水方程式を Leap-flog 法

0.6

0.6
0.4

Forest width
50m
100m
200m
400m

0.2
0.0
0

20 40
60
Wave period (min)

0.4
0.2
0.0
0

20 40
60
Wave period (min)Fig.2 津波周期と津波低減効果